Such a measuring instrument is known and is used for the internal investigation of pipes. With this kind of inspection the reflection is used for one thing to derive the wall thickness of the pipe and for another thing to derive possible defects in the wall of the pipe. The ultra-sound waves employed for this purpose have a frequency spectrum from 2.5-10 MHz. To this end the known ultrasonic measuring instrument is provided with a transducer beaming in the longitudinal direction of the instrument's housing and whose sound waves are directed at an acoustic rotatable mirror which is placed at an angle of 45.degree. with respect to the longitudinal axis of the instrument, which mirror reflects the sound waves radially through the window to the outside of the housing. The rotatable arrangement allows the pipe to be investigated over 360.degree.. To effectuate rotation the mirror is mounted on the shaft of a motor.
U.S. Pat. No. 4,255,798 discloses an ultrasonic measuring instrument in accordance with the preamble of claim 1.
One problem of the known ultrasonic measuring instrument is that the window may produce reflections which under certain circumstances obscure the measurement at the object of measuring. The objective of the invention is to reduce the window's influence on the measuring result, and in general to make it possible to obtain a truer measurement at the object of measuring, in which interfering influences stemming from the measuring instrument as such, are greatly reduced.